Field of the Invention
The present disclosure relates to methods of monitoring the concentrations of silver ion (Ag+) and complexing agent in tin-silver (SnAg) electrodeposition solutions, and analysis and process control using such methods.
Description of the Related Art
Historically, certain tin-lead (SnPb) bumps were electrodeposited onto semiconductor chips to provide electrical interconnections between various circuitry elements or between integrated circuit devices. Due to environmental and health concerns related to lead (Pb), tin-silver (SnAg) was identified as an alternative electrodeposition solution material. However, certain techniques for electrodeposition of SnAg bumps have posed challenges as a result of the different chemical properties of silver as compared to lead, e.g., deposition potential, as well as their interactions with each other in a plating bath. In particular, under most conditions, silver ions (Ag+) react with tin(II) ions (Sn2+) producing an undesirable precipitant product (SnO2):Sn2+(aq)+2Ag+(aq)+2H2O(l)→SnO2(s)+2Ag(s)+4H+(aq)  (1)
In order to prevent the undesirable reaction shown in equation (1), and/or to bring the reduction potential of silver closer to that of tin, a complexing agent (also referred to as a complexer or complexant) can be added to an electrodeposition solution to control the concentration of silver ions and/or allow silver to better deposit or adsorb onto the intended substrate. (For the purpose of the present disclosure, “complexing agent” and “complexer” are used interchangeably.)
Certain complexing agents that are used generally for tin or tin alloy electroplating and bump production are known in the art. For example, U.S. Patent Application Publication No. 2015/0267310 discloses a tin or tin alloy electroplating bath comprising an inorganic acid, an organic acid, and a water-soluble salt; a non-ionic surfactant; and leveling agents—with optional additive complexing agent components selected from thioamide compounds and non-aromatic thiol compounds. A preferred type of thioamide compound is thiourea.
In the deposition of SnAg bumps, both silver ions and complexing agent are consumed in the process. However, maintaining uniform manufacturing quality of integrated circuit devices can require a suitably constant composition, i.e., a suitably constant concentration, of reactants in the electrodeposition solution bath. Thus, it can be important to monitor the concentrations of silver ions and complexing agent in solution over the course of the electrodeposition process, and to do so in a safe and simple but accurate way.
Certain, existing methods have been problematic, whether due to measuring silver ion concentration, or measuring complexing agent concentration, or both.
Methods to measure silver ion concentration include atomic absorbance spectroscopy (AAS), inductively coupled plasma spectroscopy (ICP), or titration of silver ions by bringing the cation out of solution through precipitation of an insoluble product, commonly using iodide. However, certain AAS techniques require use of an open flame, which is not be suitable for use inside a semiconductor fabrication facility. ICP has the benefit of being an automated process, but it can require expensive and bulky analyzer equipment coupled with a pre-dilution system.
Methods to measure complexing agents include high performance liquid chromatography (HPLC) or titration of “free complexer,” i.e., complexing agent that is not bonded to silver ions, using a silver-based titrant with a silver electrode and subsequent calculation, where the total complexing agent concentration is equal to the free complexer in solution plus the complexer amount bound to silver ions. However, HPLC can be expensive and complicated, and can require use of flammable and hazardous solvents such as methanol or acetonitrile. Measurement of the complexing agent using “free complexer” titration can require two separate analysis steps, which compounds the likelihood of error, increases overall analysis time, and consumes large amounts of chemicals.
Certain methods of electrodeposition and/or using titration to monitor concentrations of metal ions and/or complexing agent during electrodeposition are known in the art. For example, U.S. Pat. No. 6,890,758 to Shalyt et al., involves determining the concentration of citrate complexing agent in an electroless cobalt or nickel plating bath by titration with a standard lanthanum nitrate solution containing a small, predetermined concentration of free fluoride ions. U.S. Pat. No. 7,273,540 to Sonada and Nakamura, discloses an electrolytic plating method using plating solutions that include tin, copper, and silver ions, and a complexing agent. The patent discloses that control of concentrations of the metal ions in the plating solution can be achieved by volumetric techniques, such as oxidation-reduction titration, chelatometric titration, and precipitation titration.
U.S. Pat. No. 8,118,988 to Shalyt et al., teaches a simple titration method involving a single copper ion titrant to determine concentrations of both copper ions and bath complexing agent in alkaline copper electroplating baths used to deposit or thicken copper seed layers on silicon wafers. In this reference, the electrolyte is a chelated copper solution, and a chelator reagent is added before the titration analysis. U.S. Pat. No. 8,920,623 to Luo et al., discloses techniques for replenishing tin and its alloying metals in an aqueous electrolytic plating bath using an acidic solution containing stannous oxide. In one example, the patent discloses an initial tin/silver alloy electroplating composition. Silver ion concentration was analyzed by AAS, and the complexing agent-1-allyl-2-thiourea—was analyzed by reverse titration.
U.S. Patent Application Publication No. U.S. 2012/0138471 teaches an electroplating apparatus that allows for continuous simultaneous electroplating of two metals with substantially different electrodeposition potentials, such as SnAg alloys. Similarly, U.S. Patent Application Publication No. U.S. 2013/0334052 discloses an alloy plating system that performs continuous electroplating while maintaining substantially constant concentrations of plating bath components for extended periods of use. The reference states that concentration of the plating bath components can be monitored using a variety of sensors and titrations without providing details.
Thus, there remains a need for safe, simple, and inexpensive methods that accurately measure the concentrations of silver ions and complexing agent during the electrodeposition of SnAg bumps.